Ordered a rear carrier from Hitchcocks, priced at £58.00. Got an email from them asking me to confirm, as the shipping would be £78.00. I asked them to cancel the order!
I wish something like this were available from Nfieldgear. This is made in England and is rated at 7.5kg (16.5 lbs). The made-in-India adjustable rear carrier at Nfieldgear is priced nice at $64.95, but doesn't properly fit the B5, and eventually cracked on both sides even though the top-case I had on it was only used for carrying rain gear and an iPad. I may order another one, anyway, but I would love to have something really sturdy.
Hitchcock's also has a set of side carriers that come with the small Givi panniers. Maybe I should find a local welding shop and have something custom made.